1. Field of the Invention
The present invention relates to an inverter device.
2. Description of the Related Art
Switching regulators or inverter devices are used to supply high voltage to various devices such as a discharge tube for large-size plasma display and a plasma generator. In general, those with an output power value of about a few watts are commonly used; however, in plasma generators or the like, inverter devices with an output voltage of a dozen kilovolts and an alternating-current (AC) output power value of tens of watts or more are used.
A general switching regulator (an AC or DC-DC converter) intermittently applies a direct-current (DC) voltage to a primary-side excitation winding of a transformer for voltage conversion by causing a switching element to switch the DC voltage on and off, and rectifies and smooths an AC current generated in a secondary-side output winding, and then outputs a DC voltage. To maintain the output voltage at a constant voltage, for example, as shown in Japanese Laid-open Patent Publication No. 2009-11144, the switching regulator detects an output voltage and generates a feedback voltage, and performs pulse-width modulation (PWM) control for controlling the ratio of an ON time to an OFF time (the duty ratio) of the switching element on the basis of the feedback voltage.
This is to compensate for lack of output power by increasing the ON width of the switching pulses if the output power has decreased, or, on the contrary, to limit the excess output power by reducing the ON width if the output power has increased, thereby controlling the output voltage to be constant.
On the other hand, an inverter device intermittently applies a DC voltage to a primary-side excitation winding of a transformer for voltage conversion by causing a switching element to switch the DC voltage on and off in the same manner as the above-described switching regulator, and directly outputs an AC voltage generated in a secondary-side output winding to a load. In this case, for example, as shown in International Publication Pamphlet No. WO 2007/060941, the inverter device detects an output current instead of an output voltage, and converts the detected current to a voltage, and then performs PWM control on the switching element.
In a case of a switching regulator whose output voltage is DC voltage, as described in Japanese Laid-open Patent Publication No. 2009-11144, the switching regulator can detect the output voltage and perform PWM control on the switching pulses for ON/OFF control of a switching element. Furthermore, the output side has a retention time of an electrolytic capacitor of a smoothing circuit; therefore, the responsiveness of the control does not become a problem. However, the output of an inverter device is AC voltage; therefore, regardless of whether the output voltage is full wave or half wave, it is difficult to control the peak value of the output voltage (the peak voltage value) to be constant.
That is, the time of the peak voltage value is at a point, and a difference in voltage level is large, so the number of elements for detecting the peak value increases, and a delay in the control response time is caused by parasitic inductance. The higher the frequency of an output voltage waveform repeated, the more prominent the influence of the delay, and the peak voltage value drops too low or rises too high. In the worst case, a resonance frequency deviation is generated, and, when resonant voltage is applied, if an excitation current flows in an ON state of the next switching cycle, energy of a residual voltage which has nowhere to go becomes an excess current, resulting in an imbalance of resonance. This may cause a failure of the switching element due to power beyond its capacity, or may cause saturation of a transformer.
In this way, in a case of an inverter device whose output is AC voltage, switching frequency is as high as tens of kilohertz, and peak voltage value of output using voltage resonance is as high as a dozen kilovolts, there are a problem of the voltage resistance of an output-voltage detecting means or components and a problem of the time to complete in addition to the above-described problem of control responsiveness. Therefore, in such an inverter device, an input supply voltage is manually set by means of a lever, and an output voltage is just adjusted in accordance with the set input voltage; in general, the value of the output voltage is not constantly monitored.
In view of these circumstances, there is a need to provide an inverter device capable of controlling the peak voltage value of AC output voltage to be constant even if input voltage changes.